Hydrostatic systems are commonly utilized for powering a wide variety of apparatus, including on work machines such as tractors used for agricultural, construction, and other applications. A particularly pertinent application for the present invention is crop cutting apparatus of agricultural harvesting machines, most particularly, disk cutting apparatus of windrowing machines. A disk cutter includes rotatably driven disks having knives extending radially outwardly from the outer circumferential edges thereof, arrayed across the width of a header propelled by a tractor. The header can be supported on the front of the tractor, or towed. The disk cutters are driven by a fluid motor of a hydrostatic system, and, for a wider header with a large number of cutters, two or more hydrostatic systems may be used. The fluid pump of the hydrostatic system will typically be located on the tractor and driven by the tractor engine, and will provide the driving fluid to the motor, through fluid lines that extend between the pump and motor, forming a fluid loop.
A typical cutting speed for a disk cutter is about 2200 to 3200 rpm. It is typically desirable for the cutters to operate at a constant or set speed, under a variety of changing conditions, which can include, but are not limited to, varying crop population and moisture content, cutting viny, thick stalked or shrubby weed infestations, occasional contact of one or more cutters with the ground, and/or varying available engine power. Closed loop speed control is typically employed for maintaining a constant cutting speed, and the pump or the motor will have a displacement that can be varied for achieving generally constant motor and cutter speed under the changing power availability and load conditions. This variable displacement will typically be controllable by a solenoid. Varying electrical current signals are utilized for operating the solenoid, for operating the cutters at the desired speed and in the desired direction.
In a typical speed control arrangement, the electrical current signals can be controlled by an operator using a switch or dial located in the operator cab, or automatically controlled, using an automatic speed control, which can have a selectable speed. The automatic speed control will monitor a speed sensor, for instance, for sensing the speed of the motor or an element driven thereby, and will vary the electrical current signal as required for maintaining the selected speed. In operation, when a speed decrease is sensed the displacement of the variable displacement pump or motor will be varied, as required, to bring the speed back to the set value.
If a speed decrease is the result of just an increased load on the cutters, for instance, as a result of conditions such as ground contact, increased crop population or density, higher moisture content or other common conditions, a displacement variation to compensate to increase cutter speed is not problematic. However, a motor speed reduction can also occur as a result of a pump speed reduction, which, because the pump is typically driven by the engine, will also reflect an engine speed reduction or droop. An engine speed reduction or droop, can occur for a variety of reasons, including an intentional one, wherein the operator reduces engine speed for any of a variety of reasons, and also a load induced one, for instance from increased demand from one or more powered systems, such as the tractor drive system, and/or the cutters themselves. A load induced engine speed reduction can be problematic, as it can degrade overall machine performance, and, if severe enough, can result in engine stalling. If the engine stalls, the cutters will also stall, and, if this occurs during cutting, the cutters can clog with crop.
A related problem that can occur in a system utilizing a variable displacement pump, is that in an automatic operating mode, if the controller attempts to increase the cutter speed, this will likely entail increasing the displacement of the pump, which will also increase the load on the engine. This can result in further engine speed droop, poor engine performance, and possibly stalling, if the load is not reduced, such as by removal of the source of the excessive load, or if the operator does not intervene, such as by increasing throttle position, raising the header, and/or slowing ground speed.
Accordingly, what is sought is an automatic speed control for a hydrostatic system, which provides one or more of the operating advantages, and overcomes one or more of the problems, set forth above.